Nevadan orogeny

The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kinds of orogenic processes occurring. During the early stages of orogenesis an "Andean type" continental magmatic arc developed due to subduction of an oceanic plate beneath the North American Plate. The latter stages of orogenesis, in contrast, saw multiple oceanic arc terranes accreted onto the western margin of North America in a "Cordilleran type" accretionary orogen. Deformation related to the accretion of these volcanic arc terranes is mostly limited to the western regions of the resulting mountain ranges (Klamath Mountain range and Sierra Nevada Mountains) and is absent from the eastern regions. In addition, the deformation experienced in these mountain ranges is mostly due to the Nevadan orogeny and not other external events such as the more recent Sevier and Laramide Orogenies . It is noted that the Klamath Mountains and the Sierra Nevada mountains share similar stratigraphy indicating that they were both formed by the Nevadan orogeny. In comparison with other orogenic events, it appears that the Nevadan Orogeny occurred rather quickly taking only about 10 million years as compared to hundreds of millions of years for other orogenies around the World (ex. Trans-Hudson orogeny).

The Nevadan Orogeny began with the formation of a continental volcanic arc due to east dipping subduction beneath the North American Plate. Continued subduction of oceanic crust transported multiple oceanic arc terranes to the western margin of North America where they were accreted onto the edge of the continent. During the accretion of the arc terranes onto North America, the sediment and crustal material between North American and the incoming arc terrane were thrust onto the continent forming ophiolite sequences that are preserved in both the Klamath and Sierra Nevada mountains. These mountain ranges are located in northern California-southern Oregon, and central California respectively. The accretion of arc terranes resulted in the generation of three distinct belts in the Sierra Nevada Mountains: the Western belt, Central belt, and Eastern Belt. The Klamath Mountains are somewhat more complex in their overall structure than the Sierra Nevada Mountains.

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